The present invention relates to a control circuit for controlling the output power of a primary-controlled switched mode power supply unit. In particular, the present invention relates to a control circuit in a primary-control switched mode power supply unit, which comprises a primary switch and a transformer with an auxiliary winding. Within the auxiliary winding, voltage pulses are induced by primary-sided switching operations, which can be taken into account when controlling the output power. Further, the invention relates to a method for operating a switched mode power supply unit of this kind. Moreover, the present invention relates to a method for controlling the output of a primary-controlled switched mode power supply unit with a transformer and a primary-sided switch, wherein the primary-sided switch can be controlled by a driver.
Switched mode power supply units are used in numerous electronic devices, in order to generate out of a mains voltage a low DC voltage which is necessary for supplying the electronic components. In many application fields switched mode power supply units are preferred to conventional power supply units with mains transformers because they have better efficiency starting from a particular performance category and have in particular a lower space requirement. The latter is in particular due to the fact that instead of the mains voltage a high-frequency alternating voltage is transformed, which can have instead of the conventional mains frequency of 50 Hz or 60 Hz for instance a frequency in a range from 20 kHz up to 200 kHz. Because the necessary number of turns of the transformer decreases inverse proportionally to the frequency, the copper losses may be reduced strongly and the transformer required will be significantly smaller.
In order to further optimize the efficiency, in particular primary switched power supply units are known, wherein the frequency which is generated on the primary-side of the high frequency transformer by means of switch, for instance a bipolar transistor, is controlled dependent on the load which is present on the secondary side of the power supply unit in order to avoid a saturation of the high frequency transformer. The feedback which is necessary for such a control scheme can be realized for instance by using a voltage tapped at an auxiliary winding as controlled variable. This is for instance shown in the European patent application EP 1 146 630 A2. The method for controlling the output current and/or the output voltage of a switched power supply as shown in document EP 1 146 630 A2, implies that with each pulse the same energy is loaded into the transformer. Each time after a fixed time with respect to the opening of the switch, the voltage at the auxiliary winding is sampled and stored by means of a sample and hold element (S & H element). The disadvantage of this method, however, is that the sample timing is of great significance for the performance of the controlling and that the form of the voltage pulses at the auxiliary winding is strongly influenced by the different operation parameters, such as input voltages, disturbances etc. That means that a fixed value for the instant of sampling limits the flexibility and range of application for such a switched mode power supply unit strongly.